Support flushing command history

[414owen]

Doesn't look like https://github.com/judah/haskeline/pull/36 will be updated, I'm happy to pick up where that PR left off.

Changes:

• History is appended instead of overwritten
• Can be enabled via the preference incAppendHistory (based on ZSH's INC_APPEND_HISTORY)

Regressions:

In order to append to history file, items are now written in the order they were entered. This is the opposite of the previous behavior, so when applications upgrade, their history up to that point will be in reverse order.

[nkartashov]

@414owen hey, sorry for disappearing, happy for you to pick it up!

[414owen]

:wave: @judah